Yes, excess leverage is a bad thing and I never suggested otherwise. The sole point I am making is that using notional value as a measure of leverage is negligent, lazy, stupid and utterly wrong. Even further, to compare the notional value of a derivative contract to world GDP is silly beyond imagining.
Let me give you a trivial example. Eurodollar futures are simple derivatives (which are probably included in the author's tally); each contract's notional is $1mln. Current open interest is arnd 7mln contracts, which means that the total notional exposure of the mkt in Eurodollars is arnd $7trn, which is larger than China's GDP. What does that tell you exactly?
EDIT: I have argued over this at some length in a couple of different ET threads.